The present invention relates to cross-flow filtration for removal of suspended and colloidal solids and/or emulsified oil from liquids, particularly, water, wastewater, industrial wastes, and industrial process streams. More specifically, the invention relates to a method and apparatus for increasing the time-averaged rate of transfer of liquids containing such solids and/or oil across a cross-flow filtration medium (referred to herein as the "time-averaged filtration flux").
In water and wastewater treatment, clarifiers and through-flow filters are conventionally used for removal of suspended and colloidal solids. Such systems have inherent disadvantages. Without preclarification, most filters are unable to handle the resulting higher solids loadings. Likewise, even with a clarifier in the process train, filtered particles continuously accumulate on and within through-flow filter media. The filter flux rate decreases with time (or headloss increases) and frequent backwashing is required to remove the accumulated solids from the filter medium. When product water is used for back-washing there is a significant net decrease in total water production. Relatively large volumes of low solids wastewater are also created which must receive some type of further handling. There is also the problem of filter breakthrough. Moreover, water quality is extremely process dependent.
Cross-flow filtration is substantially different from through-flow filtration, in that feed water is introduced parallel to the filter surface, and filtration occurs in a direction perpendicular to the direction of the feed flow. Cross-flow filtration satisfies a much wider range of applications and provides economic benefits that other conventional options do not. Cross-flow filtration systems are capable of clarification, filtration, and thickening in one process step. Equipment costs approach those of direct filtration; yet cross-flow filtration is capable of filtering streams that contain suspended solids concentrations of 10,000 mg/L or higher. Furthermore, cross-flow systems require less space than conventional systems. Cross-flow filtration systems include membrane systems such as microfiltration, reverse osmosis and ultrafiltration. The major disadvantages of the latter two membrane processes in liquids-solids separation are low flux rates and susceptibility to fouling. These liabilities ultimately translate into high system construction and operating costs. However, both of these problems have been virtually eliminated in a new method of cross-flow microfiltration utilizing thick-walled porous thermoplastic tubes sold under the trademark HYDROPERM.TM.. The filtration characteristics of these tubes combine both the "in-depth" filtration aspects of multi-media filters and the "thin-skinned" aspects of membrane ultrafilters. The porosity of HYDROPERM.TM. tubes results from the open cell reticulated structure of the tube wall. HYDROPERM.TM. tubes differ from conventional membrane ultrafilters, in that they have pore sizes on the order of several microns, wherein the length of a pore is many times that of its diameter. These tubes are described in greater detail, for example, in "HYDROPERM.TM. CROSS FLOW MICROFILTRATION", Daniel L. Comstock, et al., Neptune Microfloc, Inc. Report No. KT 7307, May 1982, and in Report No. 77-ENAS-51 of the American Society of Mechanical Engineers, entitled "Removal of Suspended and Colloidal Solids from Waste Streams by the Use of Cross-Flow Microfiltration", which reports are hereby incorporated herein by reference to the extent necessary for a thorough understanding of the background of the invention.
Feed flow is through the center of HYDROPERM.TM. tubes at a relatively low pressure, typically less than 30 psi. The filtrate is typically collected in a jacket surrounding the exterior tube wall and withdrawn therefrom by a product line. As feed flow circulates through the tube, solid particles are slowly driven with the product flow toward the tube wall. Thus, the concentration of particles in regions close to the wall steadily increases.
In cross-flow filtration systems generally, because the direction of the feed flow is tangential to the filter surface, accumulation of the filtered solids on the filtering medium is reduced by the shearing action of the flow. Cross-flow filtration thus affords the possibility of a quasi-steady state operation with a nearly constant flux when the driving pressure differential is held constant. Unfortunately, this theoretical possibility has not been achieved in practice. Thus, the problem of declining filtration fluxes has plagued conventional cross-flow filtration systems.
In general, any liquid from which suspended solids removal is desired will contain a wide range of particulate sizes, ranging in effective diameter from several microns down to colloidal dimensions. Because of the "in-depth" filtration characteristics of thick-walled, thermoplastic tubes, such as HYDROPERM.TM. tubes, particles smaller than the largest pore size of the tube may, under certain circumstances, enter the wall matrix. In any event, above a certain solids concentration in the feed, the majority of the suspended solids are retained at the inner wall of the tube and quickly form a dynamic membrane (also referred to as a "filter cake" or "sludge layer"). The dynamic membrane is, we believe, largely responsible for the filtration which subsequently occurs.
Those particles initially entering into the wall matrix ultimately become entrapped within it, because of the irregular and tortuous nature of the pore structure. As microfiltration proceeds, penetration of additional small particles into the wall matrix is inhibited by the presence of the dynamic membrane. The formation of the dynamic membrane, together with the possible clogging of the pore structure of the tube by entrapped particles, results in a decline in the filtration flux. In conventional systems, this decline is approximately exponentially related to filtration time.
In view of the fact that an increase in filtration flux will permit far more economical processing of solids laden liquids, the art has sought methods for inhibiting the above-described filtration flux decline in cross-flow filtration systems and/or for restoring the filtration flux in such systems to a higher value, after it has declined.
Various cleaning techniques have previously been investigated for restoring the filtration flux value. Such cleaning techniques have involved chemical and/or physical cleaning of the surface of the filter medium. For example, chemical solvents have been used to dissolve the layer-building filtered particles so as to yield a clean, layer-free filter surface. Hydrochloric acid and other acids are examples of solvents commonly being used. On the other hand, a simple physical cleaning technique commonly used is backflushing of the filter medium, i.e., temporary reversal of the filtrate flow direction. This cleaning technique is frequently used in conjunction with cross-flow filtration processes utilizing hollow tubular filters. Another physical cleaning technique employed in the art involves periodically increasing the recycle velocity longitudinally through the porous tubes. (See, e.g., U.S. Pat. application Ser. No. 319,066.) Higher recycle rates tend to sweep away accumulated deposits, thus minimizing the build-up of the filter cake within the tubes.
Despite the success of the above-noted cleaning schemes, the cross-flow filtration art continues to search for new techniques for increasing time-averaged filtration fluxes, in order to make cross-flow filtration processes more economical.
The significant increase in time-averaged cross-flow filtration fluxes obtained in accordance with the present invention thus constitutes a significant contribution to the cross-flow filtration art.